import nltk

wn=nltk.corpus.wordnet
result=[simxy.definition() for simxy in
        max((x.path_similarity(y),x,y)
            for x in wn.synsets('cat')
            for y in wn.synsets('dog')
            if x.path_similarity(y) #确保synsets是相关的
            )[1:]]

print(result)